1. Field of the Invention
This invention relates to a method of printing components of an electronic circuit on a base in the form of a plate by screen printing and to an apparatus used therefor. More particularly, this invention relates to screen printing method and apparatus suitable for performing positioning of the base and printing on the same in a flow production manner.
2. Description of the Prior Art
In an ordinary conventional screen printing system for printing an electronic circuit on a board, the board is placed on an XY.theta. table and is supported by being attracted thereto or by a chuck, the XY.theta. table is moved by an image processor to position the board, and printing is thereafter performed on the XY.theta. table. In a case where this conventional art is applied to screen printing, however, a problem is encountered in that printing cannot be performed while positioning is being performed or positioning cannot be performed during printing, since the board is printed while being placed on the XY.theta. table. Therefore this system cannot be designed for flow production.
Japanese Patent Unexamined Publication No. 61-152355 discloses method and apparatus in which component parts are placed on an index table and are successively handled and processed. Techniques disclosed therein are mainly intended to read image information on, e.g., the shapes of component parts by an image processor with a video camera or the like and to handle the component parts on the basis of the results of reading. Thus, the proposed techniques are not directly concerned with screen printing. The index table disclosed in Japanese Patent Unexamined Publication No. 61-152355 is a means for simply supporting and transporting boards. Data obtained by the image processor is temporarily stored in a register and is compared by calculation with reference values previously stored, thereby adjusting the position of the printing unit to that of the board. In this process, the printing unit is moved together with the board. It is therefore difficult to maintain the desired printing accuracy.
Other types of conventional art are disclosed in U.S. Pat. No. 3,543,679 and Japanese Patent Unexamined Publication Nos. 57-51466, 56-58866 and 61-216387.
U.S. Pat. No. 3,543,679 and Japanese Patent Unexamined Publication No. 57-51466 each disclose a system in which an article to be printed is supplied from one article supply position onto a circular index table and fixed to the index table and is transported to a screen printing position, printing is effected on the article at the screen printing position by a screen printing unit, and the printed article is transported to a discharge position by the index table and is then discharged from the index table.
In this process, the article is transported to the screen printing position while being positioned in place by pins or a frame and fixed to the index table. In recent printed electronic circuits, however, the printed lines are each of a decreased width or thickness and spaced from each other by decreased distance than in the prior art printed circuits. In some cases, moreover, after a printed circuit has been fixed by heat, another circuit is printed in superposed relationship to the printed circuit already made. The recent printed circuits, therefore, demand a much increased positioning accuracy which cannot be achieved by the conventional mechanical positioning means, such as the combination of pins and associated holes which inevitably involves a play therebetween. Furthermore, in the case where circuits are printed one on the other in superposed relationship with each other, the printed articles are subjected to non-uniform strains generated therein when the articles are heated to fix the printed circuits. Due to the non-uniform strains, the positions of the printed circuits relative to the positioning pin holes in the printed boards or to the outer edges thereof are varied between different printed boards. Accordingly, the use of the conventional mechanical positioning means leads to the production of inaccurate prints on articles.
Japanese Patent Unexamined Publication No. 56-58866 discloses a printing system including a printing base of a polygonal cross section which is adapted to be rotated intermittently over an angle corresponding to the angle formed by each side of the polygon about the axis of rotation which extends horizontally. A board is positioned by pins relative to the surface of the lowermost side of the polygonal printing base and attracted to and held on the surface. A plurality of boards are successively attached to successive sides of the polygonal printing base and are upwardly transported by the rotation of the printing base to a screen printing position disposed above the printing base to effect printings on respective boards thus fed to the printing unit. As in the prior art already discussed above, however, the positioning of the boards to surfaces of respective sides of the polygonal printing base also relies upon mechanical positioning means which fail to assure an accurate positioning. The prior art in question, therefore, also has a problem similar to those of the prior arts previously discussed.
Japanese Patent Unexamined Publication No. 61-126387 discloses a screen printing system which employs an image pickup tube and a monitor TV so as to assure an accurate positioning of a board by a positioning mark on the board. The system comprises two embodiments in one of which a board and a screen printing unit are moved to a positioning station. In the other embodiment of the system, image pickup tubes are provided in positioning station and printing station, respectively. Boards are placed on and moved by a table successively to the positioning station where the positioning mark on each board is picked up and displayed on the monitor TV. The position of the table is adjusted until the positioning mark displayed on the monitor TV is registered with a predetermined cursor the position of which has been set in advance by directly displaying on the monitor TV a positioning mark on a printing screen and adjusting the position of the cursor until the cursor is registered with the positioning mark on the screen displayed on the monitor TV. Then, the board is moved by the table to the printing station under the screen printing unit, so that a printing is effected on the board. This prior art suffers from a problem that the system is not operative to simultaneously carry out a positioning of a board and a printing on another board and, thus, falls short of providing satisfaction in terms of productivity.